The present invention relates generally to techniques for compressing relatively large light intensity ranges and more particularly to a technique which utilizes a specifically designed cholesteric liquid crystal type of notch filter.
There are times when it is desirable to view an object having a relatively large light intensity range and record what is viewed by means of a camera or like instrument. For example, applicant has found a need to view the crucible containing molten metal in an atomic vapor laser isotope separation (AVLIS) system. However, this molten uranium containing crucible functions as a black body having a center which is at a temperature of approximately 4000.degree. K. and an outer temperature on the order of 1000.degree. K. Thus, the temperature range across this black body is approximately 4000.degree. K. and has a correspondingly large light intensity range (nine orders of magnitude) that is much too large for a standard (two-three orders of magnitude) video camera or like instrument to receive and decipher. Therefore, if the standard viewing and/or recording equipment is to be used to view the crucible, its light intensity range must be compressed to a tolerable level.
The concept of compressing a relatively large light intensity range is not new. Heretofore, it has been done electronically, that is, by converting the incoming light to corresponding electrical signals, compressing the electrical signals, and then converting these compressed electrical signals back to light which itself is compressed relative to the incoming light. Applicant has found this "electronic" approach to light compression to be relatively complicated and expensive.
To applicant's knowledge there has been no suggestion of compressing a light intensity range entirely optically, that is, by acting on the light itself rather than converting the light to electric signals which are then compressed and converted back to light. Accordingly, applicant has described in his copending patent application Ser. No. 863,912, entitled TECHNIQUES FOR OPTICALLY COMPRESSING LIGHT INTENSITY RANGES, filed May 16, 1986 an arrangement which includes a liquid crystal notch filter in combination with an interference filter. The liquid crystal notch filter described in that application (which is incorporated herein by reference) is of the general type found in the art, that is, a filter configured to pass light at all wavelengths except for light at a relatively narrow wavelength band which defines the filter's notch. However, as described in that application, the otherwise readily available filter has been modified in a way which causes the wavelength band to vary, at least to a limited extent, with temperature. As will be seen hereinafter, the present invention also provides for a liquid crystal notch filter in combination with an interference filter to provide a system for compressing light intensity ranges. However, as will also be seen, the liquid crystal notch filter disclosed herein is one which utilizes a cholesteric liquid crystal arrangement and which responds directly to light on its incident surface to vary its notch, rather than responding to changes in temperature as in the liquid crystal notch filter described in the above-recited copending patent application.
As just indicated, the present invention utilizes a specifically designed, novel cholesteric liquid crystal arrangement in forming a specifically designed light sensitive notch filter. Their new notch filter is in turn combined with an interference filter, as will be seen, to provide the desired light compression. However, at the outset, it is to be understood that cholesteric liquid crystals generally are known in the art. See for example the article entitled LIQUID CRYSTALS AS LARGER APERTURE WAVE PLATES AND CIRCULAR POLARIZERS by Stephen D. Jacobs, as published in SPIE volume 307-polarizers and applications (1981). Also, the idea of combining cholesteric liquid crystals generally (other than to form a notch filter) with one another and other components such as one-half waveplates is described in a publication entitled CHOLESTERIC FILMS AS OPTICAL FILTERS, as published in the journal of applied physics volume 42, number 10, September 1971.
Still referring to the prior art, it will be seen that a particular embodiment of the present invention utilizes an electric field responsive cholesteric liquid crystal arrangement as part of a specific notch filter. The concept that a cholesteric liquid crystal can have its transmissive properties vary with field changes is generally disclosed in British Pat. No. 1,123,117, granted Aug. 14, 1968. However, there is no suggestion in the British Patent to provide an electric field sensitive notch filter utilizing its cholesteric liquid crystal.
Still other prior art in the field of cholesteric liquid crystals includes U.S. Pat. No. 3,114,836 which discloses a technique for varying the light scattering properties of a cholesteric liquid crystal whereby to convert a heat pattern into a visible pattern and an article entitled APPLICATION OF THE LIQUID CRYSTAL LIGHT VALVE TO REAL-TIME OPTICAL DATA PROCESSING by W. P. Bleha, et al, as published in OPTICAL ENGINEERING, volume 17, number 4, July-August 1978. This latter article describes how a photoconductor received light and alters this applied electic field across a nematic liquid crystal.